JavaScript basics data types

JavaScript is a literal scripting language. It is a dynamically typed, weakly typed, prototype-based language with built-in support for types. Therefore, JavaScript is a very basic knowledge point. This article mainly introduces how basic data types and reference data types are stored in JavaScript. Friends in need can refer to it.

Basic data structure

Stack

Stack, a linear table that only allows insertion or deletion operations in a section, is A first-in-last-out data structure.

Heap

Heap is a data structure based on hashing algorithm.

Queue

Queue is a first-in-first-out (FIFO) data structure.

Storage of data types in JavaScript

Data types in JavaScript are divided into basic data types and reference data types. One difference between them is the storage The location is different.

Basic data types

We all know that the basic data types in JavaScript are:

• String

• Number

• Boolean

• Undefined

• Null

• Symbol (ignore for now)

The basic data types are simple data segments, which are stored in stack memory.

Reference data types

The reference data types in JavaScript are:

• Array

• Object

The reference data type is stored in the heap memory, and then a reference to the actual object in the heap memory is stored in the stack memory. Therefore, operations on reference data types in JavaScript operate on references to objects rather than actual objects.

It can be understood that an address is stored in the stack memory, and this address is related to the actual value in the heap memory.

Illustration

Now, let’s try to declare a few variables:

var name="axuebin";
var age=25;
var job;
var arr=[1,2,3];
var obj={age:25};

The following figure can be used to represent the storage situation of data types in memory. :

At this time, the three basic data types name, age, job are directly stored on the stack In the memory, arr, obj only stores an address in the stack memory to represent a reference to the heap memory.

Copy

Basic data types

For basic data types, if you copy, the system will automatically create new variables Allocating a new value in stack memory is easy to understand.

Reference data types

For reference data types such as arrays and objects, there will be differences when copying:

The system will also automatically assign a value in stack memory for the new variable, but this value is just an address. In other words, the copied variable has the same address value as the original variable and points to the same object in the heap memory.

If shown, after executing var objCopy=obj, obj and objCopy have the same address value and execute the same actual object in the heap memory.

What's the difference?

When I modify obj or objCopy, it will cause another variable to change.

Why?

Why do basic data types exist on the stack, while reference data types exist on the heap?

1. The heap is larger than the stack, and the stack comparison is faster.

2. The basic data type is relatively stable and takes up relatively little memory.

3. The reference data type size is dynamic and unlimited.

4. Heap memory is unordered storage and can be obtained directly based on references.

Reference article

Understanding js memory allocation

Original value and reference value

In ECMAScript, variables can store two types of values, namely primitive values ​​and reference values.
The original value refers to the value representing the original data type (basic data type), that is, the value represented by the Undefined, Null, Number, String, and Boolean types.
Reference values ​​refer to values ​​of composite data types, that is, Object, Function, Array, and custom objects, etc.

Stack and heap

There are two structures of memory corresponding to the original value and the reference value, namely the stack and the heap.
The stack is a last-in-first-out data structure. In JavaScript, Array can be used to simulate the behavior of the stack.

Primitive values ​​are simple data stored on the stack, that is, their values ​​are stored directly at the locations accessed by variables.

Heap is a data structure based on hashing algorithm. In JavaScript, reference values ​​are stored in the heap.
The reference value is an object stored in the heap, that is to say, the value stored at the variable (that is, the variable pointing to the object, stored in the stack) is a pointer pointing to the actual object stored in the heap.

例：var obj = new Object(); obj存储在栈中它指向于new Object()这个对象，而new Object()是存放在堆中的。

那为什么引用值要放在堆中，而原始值要放在栈中，不都是在内存中吗，为什么不放在一起呢?那接下来，让我们来探索问题的答案！

首先，我们来看一下代码：

function Person(id,name,age){
this.id = id;
this.name = name;
this.age = age;
}
var num = 10;
var bol = true;
var str = "abc";
var obj = new Object();
var arr = ['a','b','c'];
var person = new Person(100,"笨蛋的座右铭",25);

然后我们来看一下内存分析图：

变量num,bol,str为基本数据类型，它们的值，直接存放在栈中，obj,person,arr为复合数据类型，他们的引用变量存储在栈中，指向于存储在堆中的实际对象。

由上图可知，我们无法直接操纵堆中的数据，也就是说我们无法直接操纵对象，但我们可以通过栈中对对象的引用来操作对象，就像我们通过遥控机操作电视机一样，区别在于这个电视机本身并没有控制按钮。

现在让我们来回答为什么引用值要放在堆中，而原始值要放在栈中的问题：

记住一句话：能量是守衡的，无非是时间换空间，空间换时间的问题

堆比栈大，栈比堆的运算速度快,对象是一个复杂的结构，并且可以自由扩展，如：数组可以无限扩充，对象可以自由添加属性。将他们放在堆中是为了不影响栈的效率。而是通过引用的方式查找到堆中的实际对象再进行操作。相对于简单数据类型而言，简单数据类型就比较稳定，并且它只占据很小的内存。不将简单数据类型放在堆是因为通过引用到堆中查找实际对象是要花费时间的，而这个综合成本远大于直接从栈中取得实际值的成本。所以简单数据类型的值直接存放在栈中。

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